The process of manufacturing silicon semiconductor wafers is well known in the industry. The principal method of manufacturing them is the Czochralski (CZ) method. Generally the process includes immersing a portion of a seed crystal such as a monocrystalline silicon crystal into molten semiconductor material, such as polycrystalline silicon and, a single crystal ingot with zero dislocations grows from the seed. The seed and growing ingot are slowly moved upwardly and extracted from the molten semiconductor material allowing the crystal to grow. Growth is continued until the proper ingot size is achieved.
The CZ method has been very effective at growing semiconductor crystals. However, as crystals have gotten larger in diameter and longer and crystal growing processes are operating at higher temperatures, manufacturing and product problems have been encountered. Further, as price competition for semiconductor wafers has increased, any cost savings that can be achieved are highly desirable.
A current method of crystal production includes holding the seed, which is in the form of an elongate rod, in a chuck which is suspended from a cable. The seed is releasably retained in the chuck so when the ingot is completed, it can be easily separated from the chuck for further processing. A typical retention device for retaining the seed in the chuck is a latch pin that is interengaged with a corresponding notch previously formed in the seed. The pin engages a generally planar notch latch surface that is tapered. The degree of taper, as seen in FIG. 8, is such as to provide a taper lock whereby friction locks the seed in place. This system has worked well until the ingots have gotten larger and therefore heavier or as the crystal growing process has been conducted at higher temperatures. The heavier the crystal and the higher the crystal growing process temperature, the higher the probability of a failure.
Two modes of failure have occurred. First, the taper lock arrangement can slip and second, the seed can break ruining the crystal.
Slippage of the ingot of as little as 0.002" can result in a scrap crystal. Such slippage results in waves in the molten semiconductor material in the furnace crucible which then creates a flaw in the crystal. Slippage appears to be the result of the formation of a coating on the chuck that holds the seed. This coating is believed to be silicon carbide which forms on the chuck surface engaged by the seed during crystal growth. The coating results in a large disparity between the static and dynamic coefficients of friction between the two parts. If the seed begins movement relative to the chuck, this disparity in the coefficients of friction will allow the ingot to slip more than if they were not as disparate. Movement will continue until the friction is increased by the taper lock effect of the latch pin against the seed which additionally increases the compressive force on the seed. The slippage causes a crystal flaw at least partly because it creates minor waves in the molten semiconductor material. The formation of silicon carbide or other compound on the surfaces currently results in a higher probability of failure requiring more frequent replacement of the chuck to maintain an acceptable risk level. The layer grows more with each use, and thus increases the probability of failure with each additional use.
If the seed breaks, the crystal is also ruined since it falls into the molten semiconductor material. A chuck C, latch pin LP and seed S currently used in the art are seen in see FIG. 8. The seed S is prone to breakage. Breakage is believed to be due at least in part to the small angle A' that the latch surface LS of the notch is positioned at, which is about 11.degree. from the longitudinal axis of the seed. Such a small angle increases the compressive force applied to the seed S by the latch pin LP. Additionally, the contact between the mating surfaces of the seed and the latch pin may encourage breaking of the seed S under load. In some cases, both are made of materials having a high modulus of elasticity which is now believed to encourage breakage because of a lack of compressibility resulting in a narrow width zone of contact.
Two ways of reducing the incident rates of these failure modes is to either make the parts larger and therefore stronger or throw the parts away after fewer or even one use. However, these are expensive alternatives but would lower the probability of failure. Thus, there is need for an improved chuck and seed for producing semiconductor ingots.